1. Field of the Invention
The invention relates to transmission shifting mechanisms for small transaxles such as used in riding lawn mowers and the like. More specifically, the field of the invention is that of small transaxles having a shift lever arrangement for changing the gear ratio of an in-line shift transmission.
2. Prior Art
A transaxle having an in-line shifting mechanism is disclosed in U.S. Pat. No. 4,966,574, assigned to the assignee of the present invention, the disclosure of which is explicitly incorporated by reference. In this arrangement, the input shaft is attached to a pulley which receives motion from a belt connected with the engine. The input shaft extends out the top of the transaxle and turns a bevel gear which is freely rotatable on the shift shaft. The bevel gear engages a spur gear on the counter shaft. The other spur gears of the counter shaft engage speed change spur gears which are freely rotatable on the shift shaft. A shifting mechanism is provided to select the appropriate spur gear on the shift shaft to attain the desired transmission ratio.
The shifter fork of the shifting mechanism moves keys within the shift shaft and thereby engages one of the speed change spur gears, with the keys being rotatably fixed to the shift shaft to rotatably couple the counter shaft and the shift shaft. Another gear of the shift shaft engages the ring gear of the differential for transferring the rotational motion of the shift shaft to the output axle shafts, which are driven by a speed determined according to the selected gear ratio.
Such a transaxle may include a detent biasing mechanism for positioning the shifter fork as disclosed in U.S. Pat. No. Re. 32,125, assigned to the assignee of the present invention, the disclosure of which is explicitly incorporated by reference. In the disclosed arrangement, the shifter of the shifting mechanism includes a detent plate with a peripheral edge having a series of detent seats. Another known detent arrangement includes seats which are located in the side of the detent plate. A spring and ball are disposed in the housing of the transaxle, positioned to be adjacent to the detent seats and arranged so that the ball is biased by the spring into one of the detent seats.
The shifter is connected to a manually operable control and provides sufficient "feel" so that the vehicle operator positively senses the shift positions. In these prior art transaxles, an end of the shifter extends out of the top of the transaxle housing and is connected to a mechanism which may be manipulated by the vehicle operator. Typically, the input shaft of the shifter has a square head which is attached to an elongated lever piece so that linkage of the vehicle may rotate the shifter. In the conventional top extending shifter, the operator shift control lever is positioned lower than the tractor seat and below the front panel of the vehicle. This may be a troublesome arrangement because the belt and pulley are disposed in the general area of the input shaft of the shifting mechanism, and other parts of the vehicle are often located in the general area above the transaxle housing.
Another known design for small tractors and riding lawn mowers is a "fender shift" arrangement wherein the operator shift lever is located at one side near the front of the vehicle and pivots on a horizontal axis which is perpendicular to the direction of movement of the vehicle. However, conventional shifter arrangements having the shifter extending out of the top of the housing are not easily compatible with many small tractors and riding lawn mowers having a "fender shift" design. This incompatibility may entail additional expense associated with adapting the linkage and shifter input shaft to fit within the allocated space, and such an adaptation may be unreliable when used with conventional transaxles on vehicles having the "fender shift" arrangement.
Another feature of conventional transaxles is that the detent biasing mechanism is located between the halves of the transaxle housing and must be carefully manipulated during assembly to ensure the shifter fork is properly oriented in relation to the detents and biasing mechanism. The detent biasing mechanism serves an important function in the transaxle by allowing the operator to positively sense the shift positions. However, providing a detent plate and biasing mechanism adds to the expense of the transaxle, both in terms of additional materials and manufacturing steps. The additional materials include the detent plate itself and the biasing mechanism. The additional manufacturing steps include forming the detent seats in each detent plate, assembling the biasing mechanism in the housing, and properly aligning the shifter fork between the housing halves.
What is needed is a transaxle shifter fork arrangement which is more suitable for some vehicles having the "fender shift" arrangement.
Also needed is a shifter fork arrangement with an improved detent arrangement.